Is this the most successful animal ever? - Nigel Hughes
A procession of segmented creatures moves
across the sea floor.
Up ahead, hundreds have begun
shedding their exoskeletons
and piling on top of one another in what
appears to be... a massive orgy.
But this is not some alien world.
It’s Earth about 500 million years ago,
when these creatures, called trilobites,
prospered.
Prevailing for around 270 million years
and encompassing more than
20,000 distinct species,
trilobites are some of the most successful
lifeforms in Earth’s history.
When they sprung into existence,
they were among the most diverse
and sophisticated organisms
Earth had yet seen.
And, as the earliest known
animals with complex eyes,
trilobites had a unique perspective
on the ancient world.
For almost all of Earth’s history before
the rise of the trilobites,
life had mostly consisted of microscopic
marine organisms.
But then, scientists think an increase
in oxygen allowed multicellular lifeforms
to extract more energy from their food
and perform more complex functions.
This then enabled the rise of carnivores,
which in turn spurred
a productive arms race,
resulting in what’s known
as the Cambrian explosion.
Within about 20 million years,
life had branched out to include most
of the animal groups we know today.
Trilobites were an important
part of this surge.
All trilobites had three lengthwise lobes,
but building off the same
basic components,
they varied greatly.
Species ranged in length from a few
millimeters to almost one meter
and some were equipped
with intriguing ornaments.
As a result, they filled many distinct
niches over the ages.
Many trilobites plowed or burrowed into
the sea floor while others swam freely.
Certain species had spines, horns,
and even protruding tridents.
And their social behavior was complex:
they came together to search for food,
find safety in numbers, migrate and mate.
In fact, their fossilized conga
lines represent
some of the first evidence
of animal group behaviour.
Like modern arthropods,
trilobites had compound eyes
composed of many tiny lenses.
But theirs were made
of the mineral calcite,
which also constituted their exoskeletons.
These lenses allowed trilobites
to form sharp images
and quickly sense changes in light.
Some had long eye stalks that helped
them peep above the muddy sea floor
as they burrowed beneath.
Others had large dragonfly-like eyes
that they probably used to scour
the ocean bottom
as they swam upside-down in dim waters.
And some trilobites had pillar-like eyes
that may have offered 360 degree views,
along with structures that provided
shade from overhead light.
Yet despite their dominance of the seas,
trilobites were vulnerable
to environmental changes and predators.
About 444 million years ago,
Earth cooled and sea levels dropped,
radically changing some of their habitats.
This was the beginning of what would prove
to be the trilobite’s long demise.
Some 20 million years later,
fish with jaws began sweeping the sea.
Trilobites developed spiky ornamentation
and neat ways of locking
their joints shut.
Many species could curl themselves
into balls,
entirely sealing their soft parts
inside their hard exoskeletons.
Then, another extinction
event shook the world.
Trilobite diversity dwindled,
and come about 360 million years ago,
only one of the 10 former
trilobite orders remained.
Finally, rapid climate change spurred
the greatest known mass extinction
event in Earth's history.
This catastrophic period finished
the trilobites off—
along with approximately 96%
of all marine species.
But trilobites left a remarkable
record behind.
Their calcite exoskeletons
made for hardy fossils
that would remain intact
for hundreds of millions of years to come.
We’ve found trilobite fossils
on every single continent—
many in unlikely environments that were
once part of the ocean floor.
Trilobites have no direct descendants,
but their evolutionary cousins
are alive and well.
And since arthropods make up over 80%
of current animal species,
we might say that, although Earth may
no longer be the planet of the trilobites,
their distant relatives
still reign supreme.
